This invention relates to a wire bonding system and, more particularly, to a wire bonding system which permits automatic bonding of wires between the bonding pads on a pellet and the corresponding lead portions.
In order to carry out bonding between leads and electrode pads formed on a semiconductor pellet, there has hitherto been adopted a method in while an operator performs the operation which observing, with a microscope, the position of the workpieces to carry out the bonding. In such a method, however there are disadvantages in that the speed of the bonding operation is rather slow, mistakes in operation are likely to occur, and the operator is subjected to fatigue.
To avoid such disadvantages, attempts to automate the bonding operation have been considered.
For instance, in the bonding method disclosed in Japanese Patent Publication 37788/72, the portion on which the bonding operation is to be carried out is projected as a magnified portion on the display surface of a cathode ray tube and the coordinates of the bonding position are read out to control the head of a bonding tool to carry out wire bonding. In accordance with the method, mentioned above, however, it is difficult to read out or determine mechanically the position of the pellet and the lead projected as magnified portion on the display surface and, if the reading-out should be carried out by an operator, it will consume a large amount of time and result in a slow down of the operation. Moreover, if the operator is to read the bonding position of the pellet and the lead, a large display surface will be necessary and, such a display surface will be too imprecise to attain good-precision bonding.
Another method is the bonding system described in "Denshi Zairyo ( Electronic Materials, a Japanese publication)", May 1973, pp 38 and 39. In this bonding system, the displacements .DELTA.x, .DELTA.y and .DELTA. .THETA. of the position on which the semiconductor pellet is fixed or soldered are measured and, from these measured values of the displacement, the suitable movement of the bonding head is calculated to carry out the wire bonding automatically. This system, however, involves the following disadvantages:
1. Since the measuring device for measuring the pellet-fixed position is united with the wire bonding device, so as to carry out both the measurement of the pellet-fixed position and the bonding at the same plate, it is impossible to measure the next position, on which another pellet is fixed, during the wire bonding operation and, accordingly, many operators are required in the case of mass production;
2. For the same reason as 1, in measuring the pellet-fixed position, an ITV camera is fixed and the work-stage having the lead frame mounted on it is moved, so that the work-stage must be returned to the initial position after bonding, if the magazine for the loader and unloader of the lead frame is separate from the work-stage;
3. On the work-stage there must be provided a measuring mechanism for the displacements .DELTA.x, .DELTA.y and .DELTA..THETA., as well as a mechanism for feeding the workpiece, thus resulting in such a heavy work stage that it cannot be moved very easily, and moreover, resulting in an inaccurate bonding because the low accuracy of the measuring mechanism on account of such as vibrations during the feeding of the workpiece.
The present invention is intended to eliminate the drawbacks as mentioned above.